1. Field of the Invention
The present invention relates to thermal protection of metal-oxide-semiconductor field-effect transistor (MOSFET) devices, and more particularly to a transistor structure with thermal protection.
2. Description of the Related Art
The MOSFET components burnt out problem is often encountered in many different applications and situations. Normally a MOSFET can be operated at a junction temperature of 150 degrees Celsius but it could burn out if the heat energy inside raises the temperature to over 300 degrees Celsius. Heat can transfer by conduction, convection or radiation. Temperature changes met during MOSFET device fabrication, storage, and operation are not often large enough to place limits on component characterization and lifetime if circuit design is suitable. But the burnt out issue is still possible to occur at some abnormal conditions of fabrication, transportation, operation and insufficient design margin of circuit including MOSFET components.
During operation, components experience individually their own internal warming and thus temperature rise. The internal warming may be due to current passing through the components and comes from, for example, internal resistance of a component or switching loss. Besides, external warming is also significant. A component is influenced by external heat from other components or the environment. For integrated devices, the space between components is extremely small or the components are actually touching one another. There is little chance for convectional cooling and the spread of heat can cover many components readily. The combination of internal and external warming can lead to thermal failures of the MOSFET components if some abnormal conditions exist.
Thermal failure is the temporary or permanent impairment of component or system functions caused by thermal disturbance or damage. Thermally enhanced failures, such as oxide wearout, fracturing, package delamination, wire bond breakage, deformation of metallization on the chip, and cracks and voids in the chip, substrate, die bond, and solder joins lead to reliability limitations. Besides, the burnt out problem is a serious thermal failure result and poses critical safety concerns since it can cause smoke, fire, and melting of plastic materials. The burnt out problem could come from cold solder, poor gate drive, MOSFET itself, etc.
Specifically, MOSFET components burnt out problem is sometimes found when the MOSFET components are used for power applications such as DC-to-DC converters, batteries, and switches. Referring to FIG. 1 as an example, a pulse-width-modulated (PWM) DC-to-DC buck circuit is shown. The switching regulator shown in FIG. 1 converts the unregulated DC input voltage Vin across the filter capacitor Cin of a previous stage to the desired regulated DC output voltage Vo. Lf and Cf constitute the output filter of the buck circuit. Thermal failures may happen to the two power transistors (pass component Mp and rectifier Mn). The pass component Mp can be a PMOS transistor and the rectifier Mn can be an NMOS transistor. To prevent MOSFET burnt out problems, conventionally overheat protection components such as thermal fuses are added to interrupt the current flow through the MOSFET components if the heating effect of the current exceeds the thermal rating of the fusible element within the fuse. If the temperature exceeds the rating of the fuse, then the heat generated by the current melts the fusible element thereby interrupting the current flow (opening the circuit).
Thermal fuses can be put in series with and in close proximity to the MOS transistors or IC chips to be protected. There are disadvantages of using thermal fuses. First, fuses are usually one-shot, that is they cannot be used repeatedly. Second, open circuit caused directly by the thermal failure of the MOSFET itself can happen with the series thermal fuse intact. Since MOSFET components burn out first and then thereby become short or open circuit, doing conventional open or short circuit test may not correctly identify the cause of the circuit failure. Third, soldering the thermal fuse in the close proximity of the component being protected or reflow process may be harmful to the component. In conclusion, putting thermal fuses affects the cost, size, contact situations, etc of MOSFET components and causes such disadvantages as described above.
For the forgoing reasons, thermal protection of the MOSFET components cannot be achieved effectively by using thermal fuses. Therefore, a solution for thermal protecting MOSFET components is needed.
Accordingly, a new solution for achieving thermal protection of MOSFET components is needed and necessary. The object of the present invention is to provide a new transistor structure, as the solution, with a thermal protection unit built inside. A new transistor structure of the present invention includes a main depletion-mode NMOSFET and a control PMOSFET, with the drain terminal of the control PMOSFET connected to the gate terminal of the main NMOSFET and the gate of the control PMOSFET connected to a thermal protection unit. The two-MOSFET structure as a whole emulates a normal NMOSFET. The source terminal of the control PMOSFET that""s not connected to the gate of the main NMOSFET acts as the gate terminal of the new transistor structure, and the drain and source terminals of the new transistor structure are the drain and source terminals of the main NMOSFET. The threshold voltage of the new transistor structure is around the threshold voltage of the main NMOSFET. Similarly, another new transistor structure is composed of a main depletion-mode PMOSFET, a control NMOSFET and a thermal protection unit and emulates a normal PMOSFET.
For the first new transistor structure described above, the control PMOSFET must be turned on, when the voltage on the gate of the new transistor structure with reference to the voltage on the source of the new transistor structure exceeds the threshold voltage of the new transistor structure. To protect the new transistor from burning out when the temperature during component operation is very high, for instance higher than about 150 degrees Celsius, the control PMOSFET should be turned off regardless of the voltage on the gate terminal of the new transistor structure.
In conclusion, using the new transistor structure, with the internal thermal protection unit properly designed, as a normal MOSFET in various applications can prevent the MOSFETs of the transistor from having thermal failures, and can eliminate the disadvantages of conventional overheat protection components such as thermal fuses.